The object of this project is to evaluate the safety, immunogenicity, and efficacy of a new Hemophilus influenzae type b (Hib) vaccine (Hib capsular polysaccharide conjugated with tetanus toxoid) developed by NICHD scientists. The Contractor shall demonstrate, through a double-masked, placebo-controlled, randomized trial, that, over a 2-4 year period of follow-up, infants receiving a 3-dose course of Hib-TT vaccine will have a lower incidence of Hib disease than infants who do not receive the vaccine. The offeror will enroll infants at age 7-10 weeks, and they will be immunized at 3 separate times, every two months. Blood will be collected from a sample of the infants at each visit, and 4-6 weeks after the last injection. A blood sample from the mothers of infants bled will also be collected at the first visit. Sera will be analyzed for Hib antigen and antibody. Information will be collected from parents and area health care providers about any reactions the child may have to the vaccine, and any other health events in the follow-up period. There will be active surveillance of Hib disease in the jurisdiction served by the study site.